The present invention relates to halogenated bisdiarylaminopolycyclic aromatic compounds and polymers thereof and devices made using these polymers.
Conjugated polymers such as polyfluorenes, polythiophenes, polyphenylenes, and poly(p-phenylene vinylenes) are useful as semiconducting layers for light emitting diode devices. For example, fluorene based homopolymers are known to have a high photoluminescent capacity, which is a necessary prerequisite for electroluminescent (EL) applications. However, the homopolymers are not particularly suitable for electroluminescent devices (also known as polymeric light emitting diode devices or pLEDs) because of the inefficiency with which electrons are transferred from the homopolymers to the anode. This sluggishness of electron removal, also known as hole injection, to form radical cations (holes) in the polymer backbone is presumably due to an energy mismatch between the highest occupied molecular orbital (HOMO) of the homopolymer and the work function of the anode. This mismatch has been addressed to some degree by Woo et al. in U.S. Pat. No. 6,309,763 by incorporating triarylamines into the backbone of a polymer that contains structural units of a 9,9-dialkyl-fluorene-2,7-diyl. The presence of triarylamine repeat units have been found to increase the efficiency of hole injection from the anode to the polymer, thereby improving the efficiency of the device. Similarly, Hsieh in U.S. Pat. No. 5,879,821 describes the incorporation of triarylamines into a conjugated polymer to make a more efficient conjugated charge transport polymer.
Radical cations (holes) formed in the polymer backbone as a result of hole injection from the anode combine with radical anions (electrons) formed through electron injection from the cathode to the lowest unoccupied molecular orbital (LUMO) of the polymer to create excited states (excitons). These excitons then undergo radiative relaxation to the ground state and emit light at a wavelength corresponding to the band gap of the polymer. The efficiency and lifetime of the device, therefore, depends on the ability of the injected electrons and holes to “find” each other and recombine to form excitons. Thus, although the introduction of triarylamines into the backbone of conjugated polymers has improved the efficiency of hole injection from the anode to the polymer, there is a further need in the art to improve the efficiency of propagation or transport of holes and electrons through the polymer layer to further increase the lifetime and efficiency of the device.